1. Field of the Invention
The present invention relates to an apparatus for controlling convergence of a projection television and a method for controlling the same, and more particularly to an apparatus for controlling the convergence capable of compensating for the tilt of image influenced by the earth's magnetic field, and the method for controlling the same. The present application is based on Korean Patent Application No. 2001-48070, filed Aug. 9, 2001, which is incorporated herein by reference.
2. Description of the Related Art
A projection television using a CRT or a liquid crystal display displays an image signal as a visually recognizable picture by projecting colors of R, G, and B (Red, Green, Blue) on a screen. The position of a projecting cathode ray is precisely controlled since the deflection of the cathode ray is adjusted by a magnetic field. Therefore, the deflection degree of the cathode ray is under the direct influence of the magnetic field.
In general, when the projection television is manufactured in a production line, the degree of deflection of the cathode ray is adjusted so that the cathode ray can be precisely projected to a desired position on the CRT screen.
However, the magnitude of the earth's magnetic field at the position where the projected television is installed and used is different from that of the earth's magnetic field at the position of the production line in accordance with a region and a direction. Accordingly, the change of the magnitude of the earth's magnetic field affects the precise forming of the picture of the projection television. In other words, the change of the earth's magnetic field causes a distortion in the deflection degree of the cathode ray, thus the exact picture cannot be formed on the CRT screen. Therefore, the projection television has a built-in circuit for compensating by itself for the influence of the change of the earth's magnetic field, and the compensating circuit is called a convergence control circuit.
FIGS. 1 through 3 are views for describing a conventional method for controlling the convergence.
As shown in FIG. 1, a screen 20 for displaying the picture formed by the CRT is installed on the front side of a case 10 of the projection television. Predetermined reference patterns PH and PV are displayed on the screen 20, and a plurality of optical sensors S1, S2, S3, and S4 are disposed on the case 10.
The reference patterns consist of a horizontal reference pattern PH displayed as a horizontal line, and a vertical reference pattern PV displayed as a vertical line. The horizontal reference pattern PH moves from an upper part of the screen 20 towards a lower part of the screen 20 as indicated by a vertical arrow, and the vertical reference pattern PV moves from a left part of the screen 20 to a right part of the screen 20 as indicated by a horizontal arrow.
The optical sensors S1, S2, S3, and S4 are each disposed at an upper side, a left side, a lower side, and a right side of the screen 20. Respective positions of the optical sensors S1, S2, S3, and S4, that are (x1, y1), (x2, y2), (x3, y3), and (x4, y4) are recorded in a microprocessor (not shown) installed in the projection television. The optical sensors S1, S2, S3, and S4 measure a luminance of the reference patterns PV and PH moving horizontally and vertically, respectively. In this situation, a first sensor S1 and a third sensor S3 measure the luminance of the vertical reference pattern PV, and a second sensor S2 and a fourth sensor S4 measure the luminance of the horizontal reference pattern PH.
FIGS. 2 and 3 show a voltage output from the first optical sensor S1, in other words, a voltage value of the result after measuring the luminance of the vertical reference pattern PV. As shown in FIG. 2, when the convergence is controlled as the magnitude of the earth's magnetic field in the production line is considered, the output voltage of the first optical sensor S1, the result of the measurement of the luminance of the vertical reference pattern PV, has a maximum value at the position of x1 in a horizontal direction. However, as the degree of the influence of the earth's magnetic field is changed by the change of location of the projection television, the position of the reference patterns is somewhat deviated from x1 by an amount Δx1 as shown in FIG. 3. Thus the output voltage of the first optical sensor S1 has the maximum value at the position deviated from x1 by Δx1 in the horizontal direction. The microprocessor in the projection television uses the changed value to judge the magnitude of the earth's magnetic field, and compensates for the picture distorted for the influence of the earth's magnetic field, on the basis of the judged value. Accordingly, an exact image is formed on the screen of the projection television.
However, according to the conventional convergence control method, only the positional error in the vertical direction and the horizontal direction is compensated as described above, and accurate convergence control is limited. In other words, the horizontal reference pattern PH and the optical sensors S2 and S4 for measuring the change of the luminance of the horizontal reference pattern PH can be used only for compensating for the positional error in the vertical direction. The vertical reference pattern PV and the optical sensors S1 and S3 for measuring the change of the luminance of the vertical reference pattern PV can be used only for compensating for the positional error in the horizontal direction. Therefore, when the influence of the earth's magnetic field causes distortion in an angular direction, for example, when the vertical reference pattern PV is displayed tilted at a predetermined angle with respect to the standard position, there is no way to compensate for the tilted picture.